Abstract Using data acquired during an observational campaign carried out at the THEMIS telescope in IPM mode, coordinated
with other ground- and space-based instruments (IOACT, TRACE, EIT/SOHO, MDI/SOHO), we have analyzed the first
evolutionary phases of a recurrent active region (NOAA 10050), in order to study the morphology and dynamics of its
magnetic structures during their emergence and early development. The main result obtained from this analysis concerns
the dynamic evolution of the arch filament system (AFS) crossing the polarity inversion line: the line of sight
velocities determined from Doppler measurements confirm that the loops forming the AFS show an upward motion at their
tops and a downward motion at their extremities, but also indicate that the upward motion decreases while the active
region develops. Moreover, it has been found that, within the limits of the temporal cadence and spatial resolution of
the instruments used, the first evidence of the active region formation is initially observed in the transition region
and lower corona, and later on (i.e. after about 6 h) in the inner layers (chromosphere and photosphere). Another
interesting result concerns the analysis of the magnetograms, indicating that the initial increase in the magnetic
flux seems to be synchronous with the appearance od the active region appearance in the transition region and lower
corona, and that the rate of increase of the magnetic flux during the formation of the active region is not constant,
but is steeper at the beginning (i.e. during the first 150 h) than in the following period. All these results may
indicate the presence of some mechanism that decelerates the magnetic flux emergence as more and more flux tubes rise
towards higher atmospheric layers. Finally, we would like to stress the observed asymmetries between the preceding and
the following sides of NOAA 10050: the p-side is more extented than the f-side, the p-side moves forward from the
initial outbreak position much faster than the f-side recedes; the AFS f-side exhibits higher downflows than the
p-side.